Carbon/carbon (C/C) composites reinforced with carbon nanotube are tested for their application to primary aircraft structures using ultrasonic testing. Ultrasonic testing is the most widely used and most powerful procedure for inspecting composites. Composites are inhomogeneous and anisotropic materials with an extremely high sound attenuation. Microscopic observation shows the defects such as cracks, delaminations and debondings developed during the fabrication process. In this case, we know that conventional ultrasonic pulse-echo C-scan technique is not applicable. This paper describes an ultrasonic time-of-flight (TOF) C-scan technique for health monitoring of C/C composites. The result of the TOF C-scan image clearly suggests that a high amount of voids, cracks and delaminations causes the poor mechanical property after fabrication. This non-uniformity results from improper process control. Quality control is especially difficult when discontinuities or undesired anisotropy occur during fabrication or when the material contains local resin richness, reinforcement discontinuities, delamination or porosity in critical areas. However, we still have not yet found the most effective frequency, type of transducer, or the distance between transducer and sample for achieving the best result. Nevertheless, we show that the ultrasonic TOF C-scan technique can become a better method for the quantitative characterization of C/C composites.